This section provides background information related to the present disclosure which is not necessarily prior art.
Once a natural gas or oil well is drilled, the well must be completed to allow for the flow of petroleum or natural gas out of the formation and up to the surface. This process includes strengthening the well hole with a casing structure and then installing equipment to ensure efficient flow of natural gas or oil out of the well. The well casing consists of a series of metal tubes installed in the freshly drilled hole. The casings strengthen the sides of the well hole, ensures that no oil or natural gas seeps out of the well hole as it is brought to the surface, and keeps other fluids or gases from seeping into the formation through the well. Often times, there are multiple different layers of casings, including a conductor casing which is typically installed first prior to drilling with the drilling rig. The conductor casing is installed to prevent the top of the well from caving in and to help in the process of circulating the drilling fluid up from the bottom of the well. This casing is sometimes between 16-20 inches in diameter and is cemented into place before filling begins. A surface casing is the next type of casing to be installed. It can be anywhere from a few hundred to two-thousand feet long and is smaller in diameter than the conductor casing. When installed, the surface casing fits inside the top of the conductor casing. The primary purpose of the surface casing is to protect fresh water deposits near the surface of the well from being contaminated by leaking hydrocarbons or salt water from deeper underground. It also serves as a conduit for drilling mud returning to the surface and helps protect the drill hole from being damaged during drilling. The surface casing, like the conductor casing, is cemented into place.
An intermediate casing is usually the longest section of casing found in a well. The primary purpose of the intermediate casing is to minimize the hazards that come along with subsurface formations that may affect the well. These include abnormal underground pressure zones, underground shale, and formations that might otherwise contaminate the well, such as underground salt water deposits. In many instances, even though there may be no evidence of an unusual underground formation, the intermediate casing is run as insurance against the possibility of such a formation affecting the well. Liner strings are sometimes used instead of the intermediate casing. Liner strings are commonly run from the bottom of another type of casing to the open well area.
A production casing, alternatively called the oil string or long string, is installed last and is the deepest section of casing in a well. This is the casing that provides a conduit from the surface of the well to the petroleum-producing formation. The size of the production casing depends on a number of considerations, including the lifting equipment to be used, the number of completions required, and the possibility of deepening the well at a later time.
The wellhead consists of the pieces of equipment mounted at the opening of the well to manage the extraction of hydrocarbons from the underground formation. It prevents leaking of oil and natural gas out of the well, and also prevents blowouts caused by high pressure. Formations that are under high pressure typically require wellheads that can withstand a great deal of upward pressure of the escaping gases and liquids. These wellheads must be able to withstand pressures of up to 20,000 pounds per square inch. The wellhead typically consists of three components, the casing head, the tubing head, and the christmas tree.
The casing head consists of heavy fittings that provide a seal between the fitting and the surface of the casings. The casing head also serves to support the entire length of casing that is run all the way down the well. This piece of equipment typically contains a gripping mechanism that ensures a tight seal between the head and the casing itself. The tubing head is much like the casing head; it provides a seal between the tubing, which is run inside the casing, and the surface. Like the casing head, the tubing head is designed to support the entire length of the casing, as well as provide connections at the surface which allow the flow of fluids out of the well to be controlled. The christmas tree is the piece of equipment that fits on top of the casing and tubing heads, and contains tubes and valves that control the flow of hydrocarbons and other fluids out of the well. It commonly contains many branches and is shaped somewhat like a tree, thus its name “christmas tree.” The christmas tree is the most visible part of a producing well, and allows for the surface monitoring and regulation of the production of hydrocarbons from a producing well. As previously noted, the components of the casing head provide a tight seal between the casing and the surface of the inner casings.